Large Scale Membrane Separating Device

ABSTRACT

A large scale membrane separating installation wherein a cleaning chemical liquid flows to membrane units through a header and branch pipes. A cleaning chemical liquid distribution unit comprises the header and the branch pipes having a diameter smaller than that of the header. The cleaning chemical liquid distribution unit comprises a pipeline configuration uniformly distributing the cleaning chemical liquid supplied to the membrane units.

FIELD OF THE INVENTION

The present invention relates to a large scale membrane separatinginstallation, and to a technique of, when cleaning a membrane unit witha chemical liquid, efficiently uniformly distributing and contacting thechemical liquid to and with the membrane unit.

BACKGROUND OF THE INVENTION

In conventional submerged membrane separating installations located in areaction vessel, a membrane unit constituting a basic unit has aplurality of membrane cartridges installed in one casing. As known fromJapanese Patent Laid-Open No. 9-52026, in small scale facilities using asmall number of membrane cartridges, during cleaning with a chemicalliquid, the cleaning chemical liquid is supplied to each of the membranecartridges by gravity injection.

However, in large scale facilities, a plurality of membrane units isarranged in a line or in a plurality of lines depending on the increasedvolume of the reaction vessel. In this case, when the cleaning chemicalliquid is supplied to each membrane cartridge by gravity injection, thetime required for the supply operation increases to reduce the operationtime of the facility and thus the throughput of the facility.

Thus, the inventors have contrived a method of supplying a predeterminedamount of cleaning chemical liquid in a short time using a pump as shownin FIG. 19.

In FIG. 19, in the reaction vessel, a plurality of membrane units 1 isarranged in a line. In each of the membrane units 1, a plurality ofmembrane cartridges (not shown) are arranged parallel to one another.Each of the membrane cartridges has filtration membranes like flatmembranes located on respective surfaces of a filter plate so as to forma permeated liquid channel between the filter plate and each of thefiltration membranes. The permeated liquid channel is in communicationwith a collecting header via a tube.

A header 2 is located along a direction in which the membrane units 1are arranged. The header 2 forms a channel with a diameter (largediameter) required to ensure a smooth flow. Branch pipes 3 branches fromthe header 2 and connect to collecting headers of the respectivemembrane units 1 via valves 4. Each of the branch pipes 3 forms achannel having a smaller diameter than the header 2. Each of thecollecting headers is provided for a casing of the correspondingmembrane unit 1.

The header 2 is located at a position of a predetermined height h1 froma liquid level in the reaction vessel. Furthermore, the header 2 has apressure release valve 5 located beside a terminal thereof in a flowdirection at a position of a predetermined height h2 from the header 2.

The header 2 is in communication with a filtrate recovery pipeline 6 ata start point thereof (inlet) in the flow direction. The filtraterecovery pipeline 6 is in communication with a treated water tank 7 andhas a first selector valve 8 and a plurality of treated water pumps 9interposed in the middle thereof. Furthermore, cleaning liquid supplypipeline 11 is connected to the filtrate recovery pipeline 6 via asecond selector valve 10.

The cleaning liquid supply pipeline 11 is connected to a submerged pump12 provided in the treated water tank 7 and has an agitator 13interposed in the middle thereof. A cleaning chemical liquid supplypipeline 14 is connected between the submerged pump 12 and the agitator13. The cleaning chemical liquid supply pipeline 14 is in communicationwith a chemical tank 15 and has a transfer pump 16 interposed in themiddle thereof.

The effects of this configuration are described below. During afiltration operation, the valve 4 and the first selector valve 8 areopen. The pressure release valve 5 and the second selector valve 10 areclosed. In this condition, the treated water pump 9 is driven to exert asuction pressure on the membrane cartridges in each of the membraneunits 1. The suction pressure is used as a driving pressure to subject aliquid in the reaction vessel to a membrane separation treatment usingthe membrane cartridges.

A filtrate having permeated the filtration membrane in each membranecartridge flows through the tube (not shown) into the collecting header(not shown). The filtrate further flows through the branch pipe 3 to theheader 2 and then through the filtrate recovery pipeline 6 into thetreated water tank 7.

During a backwashing operation, the valve 4, the pressure release valve5, and the second selector valve 10 are open. The first selector valve 8is closed. In this condition, the submerged pump 12 is driven to supplytreated water in the treated water tank 7 to the header 2. The transferpump 16 is driven to supply a cleaning chemical liquid to the header 2together with the treated water.

The cleaning chemical liquid is mixed into the treated water in theagitator 13, where the concentration of the cleaning chemical liquid isadjusted to a predetermined value. The cleaning chemical liquid thenflows through the cleaning liquid supply pipeline 11 into the header 2.The concentration is adjusted by controlling the submerged pump 12 andthe transfer pump 16. The cleaning chemical liquid having flown into theheader 2 flows from the header 2 through the branch pipes 3 to thecollecting headers of the respective membrane units 1. The cleaningchemical liquid then flows into the respective membrane cartridges viathe tubes.

In the above-described operation, when the cleaning chemical liquid isinjected into the plurality of membrane units 1, an appropriate amountof cleaning chemical liquid needs to be injected into each of themembrane units 1. However, the cleaning chemical liquid is not alwaysuniformly injected into the respective membrane units 1.

When the cleaning chemical liquid is injected into the header 2, thepressure inside the header 2 increases. If the pressure increasesexcessively and the facility has no equipment such as a valve which canreduce the increased pressure, the filtration membranes in the membranecartridges may be broken by the pressure acting on the membranecartridges through the branch pipe 3, the collecting header, and thetube or by the inflow of a large amount of cleaning chemical liquidresulting from the pressure. Furthermore, the pressure inside the header2 is also a cause of the nonuniform injection of the cleaning chemicalliquid.

It is possible to provide a structure that uniformizes the amount ofcleaning chemical liquid flowing through the branch pipes 3 by usingpressure reducing valves (flow regulating valves) provided in the branchpipes 3 to cause a pressure loss while increasing the pressure in theheader 2. However, in this case, the resultant flow rate of the chemicalliquid may often be lower than the required value. Purging a gas flowingback from membrane units is also impossible.

Furthermore, when the membrane units are connected to the branch pipes3, the injected chemical liquid remaining in the membrane units maynon-uniformly flow out and leak from membrane surfaces in the membraneunits due to an uneven pressure in the branch pipe. This may alsoprevent the chemical liquid remaining in the membrane units from beingkept uniform.

Additionally, when the chemical liquid is supplied by gravity, a pump,or the like in the above-described configuration, a water head islimited and needs to be at most 100 kPa. In particular, if each of themembrane cartridges comprises a filtration membrane like a flatmembrane, the water head needs to be at most 40 kPa (preferably at most20 kPa).

The present invention solves these problems. An advantage of the presentinvention is a large scale membrane separating installation that cansimultaneously and uniformly distribute and contact the cleaningchemical liquid to and with the membrane cartridges in the plurality ofmembrane units.

SUMMARY OF THE INVENTION

The present invention provides a large scale membrane separatinginstallation including a plurality of membrane units arranged in areaction vessel and a plurality of branch pipes branching from a headerand connected to collecting headers of the respective membrane units sothat a cleaning chemical liquid flows through the header and the branchpipes to the respective membrane units, wherein a cleaning chemicalliquid distribution unit includes the header and the branch pipes eachhaving a smaller diameter than the header, and the cleaning chemicalliquid distribution unit includes a pipeline configuration uniformlydistributing the cleaning chemical liquid to be supplied to the membraneunits.

In this configuration, during a cleaning operation with the chemicalliquid, a predetermined flow rate of cleaning chemical liquid flowsthrough the header. The cleaning chemical liquid flows through theheader and the branch pipes to the plurality of membrane units. At thistime, the cleaning chemical liquid distribution unit uses an adjustingfunction provided by the pipeline configuration to adjust a first forceresulting mainly from a pressure head and a second force resultingmainly from a velocity head. The first force acts as a force that pushesthe cleaning chemical liquid in the header toward an inlet of each ofthe branch pipes. The second force acts as a force that sweeps away thecleaning chemical liquid in the header in a pipe axis direction.

Here, when the header is positioned above a liquid level in the reactionvessel, the cleaning chemical liquid flows in a non-full condition, thatis, flows with a gas phase present in the upper area of the header. Thecleaning chemical liquid thus flows through the branch pipes to thecollecting headers of the respective membrane units. Moreover, touniformly hold the chemical liquid injected into the branch pipes, evenin membrane units each located at the tip of the corresponding branchpipe, it is necessary to make the pressure in the branch pipes uniformeven after the cleaning chemical liquid has passed through the branchpipes.

At this time, to allow the cleaning chemical liquid to flow uniformlyinto the branch pipes, it is necessary to uniformly apply the supplypressure in the header to the inlets of the branch pipes. Factorsdetermining the pressure (hereinafter referred to as the branch pipeinlet supply pressure) include the amount of cleaning chemical liquidsupplied and flowing into the header, the supply pressure (the pressureof the pump or the water head in a vessel from which the cleaningchemical liquid is fed), the flow velocity of the cleaning chemicalliquid flowing through the header, the ratio of the header diameter andthe branch pipe diameter, the resistance of the pipeline, and gravity.

A detailed description will be given below. FIGS. 1 and 2 show aconfiguration similar to that shown in FIG. 19. In FIG. 2, referencenumeral 1 a denotes an air diffusing pipe in the membrane unit 1.Reference numeral 1 b denotes a blower. FIG. 3 is a diagram as viewed inthe direction of arrow a-a in FIG. 1 and shows three cases in which thebranch pipes 3 have different structures. FIG. 4( a), (b) and (c) aregraphs showing the flow rate of the cleaning chemical liquid flowingfrom each of the branch pipes 3 to the corresponding membrane unit 1 ineach of the configurations shown in FIG. 3. In the graph, for referencenumerals identifying the branch pipes 3, the branch pipe 3 locatedclosest to the start point (inlet) of the header 2 in a flow directionis denoted by NO. 1. The branch pipe 3 located closest to the terminalof the header 2 in the flow direction is denoted by NO. 8.

In the above-described configuration, when the branch pipes 3 areconnected to the header 2 in a horizontal direction, the flow velocityof the cleaning chemical liquid, one of the factors uniformizing thebranch pipe inlet supply pressure, has a greater impact than the otherfactors.

That is, as shown in FIG. 4( a), if the flow velocity of the cleaningchemical liquid is high, the cleaning chemical liquid has moredifficulty flowing into the branch pipes 3 located closer to the startpoint (inlet) of the header. The cleaning chemical liquid flows moreeasily into the branch pipes 3 located closer to the terminal of theheader 2.

This phenomenon is attributed to the following. That is, at the startpoint of the header, the second force (resulting mainly from thevelocity head) acts more dominantly than the first force (resultingmainly from the pressure head). The force sweeping away the cleaningchemical liquid in the pipe axis direction of the header 2 dominates theforce pushing the cleaning chemical liquid toward the inlet of eachbranch pipe. On the other hand, at the terminal of the header, the flowof the cleaning chemical liquid impacts the terminal of the header 2 toincrease the pressure head.

When the branch pipes 3 are arranged so as to hang from the header 2 ina vertical direction, the pressure of the cleaning chemical liquid, oneof the factors uniformizing the branch pipe inlet supply pressure, has agreater impact than the other factors.

That is, as shown in FIG. 4( b), the cleaning chemical liquid flows moreeasily into the branch pipes 3 located closer to the start point (inlet)of the header. The amount of cleaning chemical liquid reaching theterminal of the header 2 decreases consistently with the distance to theterminal of the header 2, making it more difficult for the cleaningchemical liquid to flow into the branch pipes 3.

This phenomenon is attributed to the following. That is, in addition tothe pressure head resulting from the supply pressure of the pump, thegravity acts as a force pushing the cleaning chemical liquid toward theinlet of each branch pipe. Thus, at the start point of the header, thefirst force (resulting mainly from the pressure head) acts moredominantly than the second force (resulting mainly from the velocityhead). The force pushing the cleaning chemical liquid toward the inletof the branch pipe dominates the force sweeping away the cleaningchemical liquid in the pipe axis direction of the header 2. As a result,most of the cleaning chemical liquid flows into the branch pipes 3located closer to the start point of the header, reducing the amount ofcleaning chemical liquid reaching the terminal of the header 2.

Thus, according to the present invention, with the “gravity” and“velocity head” taken into account, the cleaning chemical liquiddistribution unit includes the pipeline configuration adjusting thefirst force and the second force. This allows the cleaning chemicalliquid to flow uniformly to the plurality of membrane units through theheader and the branch pipes.

A specific configuration of the cleaning chemical liquid distributionunit is as follows. The header is located along a direction in which themembrane units are arranged and along a horizontal direction. Mountingaxes of the branch pipes are arranged parallel to one another. Theheader is positioned above a liquid level in the reaction vessel. Eachof the branch pipes is inclined with a falling gradient from the headertoward the collecting header at a predetermined angle.

In this configuration, for example, the branch pipes 3 are inclined at45° to the header 2 to adjust the “gravity” and “velocity head” so as touniformize the branch pipe inlet supply pressure.

That is, the rate of the gravity in the total force of the gravity andthe “pushing force”, the first force, to which the gravity is added, isadjusted by the inclination of the branch pipes, to inhibit the flow ofan excessive amount of cleaning chemical liquid into the branch pipeslocated closer to the start point of the header, while the cleaningchemical liquid is swept away toward the terminal of the header by the“sweeping force”, the second force. Thus, as shown in FIG. 4( c), thebranch pipe inlet supply pressure is uniformized such that the cleaningchemical liquid flows uniformly into all the branch pipes 3.

Furthermore, in this configuration, each branch pipe has no valvereducing the pressure. The branch pipe is not full of the cleaningchemical liquid, but a gas phase is present therein. The branch pipe isalso in communication with the air, and all the branch pipes are kept ata fixed pressure, the atmospheric pressure.

Thus, the chemical liquid remaining inside the membrane units can bekept uniform from membrane surfaces in the membrane units each connectedto the tip of the corresponding branch pipe.

That is, since each of the branch pipes is inclined with the fallinggradient from the header toward the collecting header at thepredetermined angle, the cleaning chemical liquid can be uniformlydistributed to and contacted with the plurality of membrane unitsthrough the header and the branch pipes.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located along thedirection in which the membrane units are arranged and along thehorizontal direction. The mounting axes of the branch pipes are arrangedparallel to one another. The header is positioned above the liquid levelin the reaction vessel. Each of the branch pipes has an overflow portionat a predetermined height position above the header.

In this configuration, the pressure head in the header is set to a valueat which the cleaning chemical liquid is lifted to the overflow portionof each branch pipe. In other words, the predetermined height positionof the overflow portion, formed in the branch pipe, is set on the basisof the pressure head generated in the cleaning chemical liquid in theheader by the supply pressure of the cleaning chemical liquid (thesupply pressure of the pump or the like) so that when the cleaningchemical liquid flows through the overflow portion, a gas phase ispresent in an upper area of the overflow portion.

During cleaning with the chemical liquid, the cleaning chemical liquidflows so as to fill the header. The cleaning chemical liquid flowingfrom the header to the branch pipes overflows through the overflowportion with a gas phase present in the upper area of the overflowportion and flows to the collecting headers of the respective membraneunits.

At this time, the interior of the header is full of the cleaningchemical liquid to inhibit a possible nonuniform branch pipe inletsupply pressure resulting from the velocity head. Furthermore, thegravity does not act as the “pushing force” pushing the cleaningchemical liquid toward the inlet of the branch pipe. Instead, only thepressure head (the supply pressure of the pump or the like) acts as the“pushing force”, the first force. The pressure head lifts the cleaningchemical liquid to the overflow portion against the gravity to allow thecleaning chemical liquid to overflow.

Therefore, since the branch pipe has the overflow portion at thepredetermined height position above the header, the cleaning chemicalliquid can be uniformly distributed to and contacted with the pluralityof membrane units through the header and the branch pipes.

Furthermore, an air open portion may be provided in the overflow portionof each branch pipe. In this case, the overflow portion is open to theair at the air open portion. Thus, when the cleaning chemical liquidoverflows through the overflow portion, the gas phase in the overflowportion is prevented from causing air lock. The cleaning chemical liquidflows naturally downward through the branch pipe under the atmosphericpressure to the collecting header of the corresponding membrane unit.The cleaning chemical liquid can thus be uniformly distributed to andcontacted with the plurality of membrane units through the header andthe branch pipes.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located along thedirection in which the membrane units are arranged and along thehorizontal direction. The mounting axes of the branch pipes are arrangedparallel to one another. The header is positioned below the liquid levelin the reaction vessel. Each of the branch pipes is connected to theheader below the liquid level.

In this configuration, the cleaning chemical liquid supplied to theheader during cleaning with the chemical liquid flows to each branchpipe so as to fill the header. The cleaning chemical liquid then flowsto the collecting header of each membrane unit through the correspondingbranch pipe.

At this time, since the header is positioned below the liquid level inthe reaction vessel, the interior of the header is full of the cleaningchemical liquid. This inhibits a possible nonuniform branch pipe inletsupply pressure resulting from the velocity head.

Furthermore, the water head corresponding to the distance from theliquid level to the header acts as a back pressure. Consequently, thegravity does not act as the “pushing force” pushing the cleaningchemical liquid toward the branch pipe inlet. Thus, only the pressurehead (the supply pressure of the pump or the like) acts as the “pushingforce”, the first force, to push the cleaning chemical liquid toward thebranch pipe inlet. The cleaning chemical liquid can thus be uniformlydistributed to and contacted with the plurality of membrane unitsthrough the header and the branch pipes.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located along thedirection in which the membrane units are arranged and along thehorizontal direction. The mounting axes of the branch pipes are arrangedparallel to one another. The header is positioned at or above the liquidlevel. The header is divided into a plurality of short pipelines each ofwhich is located for every predetermined number of membrane units andeach of which is in communication with a cleaning chemical liquid supplysource at a central part thereof in a pipe axis direction.

In the above-descried configuration, the cleaning chemical liquidsupplied to each header during cleaning with the chemical liquid flowsthrough the branch pipes to the collecting headers of the respectivemembrane units.

At this time, since each of the short pipes of the header is present forthe corresponding one of the membrane units and each short pipe is incommunication with the cleaning chemical liquid supply source at thecentral part thereof in the pipe axis direction, the liquid level andvelocity head in the short pipes are inhibited from being nonuniformlydistributed. The cleaning chemical liquid can thus be uniformlydistributed to and contacted with the plurality of membrane unitsthrough the short pipes of the header and the branch pipes.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located at an equaldistance in the vertical direction from the plurality of membrane unitscircularly arranged in the reaction vessel. The branch pipes branchingfrom the header are radially arranged. The header is positioned at orabove the liquid level.

In this configuration, the cleaning chemical liquid supplied to theheader during cleaning with the chemical liquid flows through the branchpipes to the collecting headers of the respective membrane units.

At this time, since the cleaning chemical liquid flows from the headerinto the radially branching branch pipes, the possible nonuniformdistribution of the velocity head in the header is avoided. The cleaningchemical liquid can thus be uniformly distributed to and contacted withthe plurality of membrane units through the header and the branch pipes.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located along thedirection in which the membrane units are arranged and along thehorizontal direction. The mounting axes of the branch pipes are arrangedparallel to one another. The header is in communication with thecleaning chemical liquid supply source at the inlet of the header at oneend thereof in the pipe axis direction. The header has a pressurecontrol device provided at the other end thereof in the pipe axisdirection to maintain a pressure at less than or equal to a set value.

In this configuration, if an excessive amount of cleaning chemicalliquid is injected into the header to exert an excessive pushing forcepushing the cleaning chemical liquid toward the inlet of each branchpipe, the pressure control device is activated to release the excessivepressure, preventing the excessive pressure from acting on the membraneunits.

The pressure control device includes a pressure sensing device, a valvedevice that is in communication with the header, and a control devicethat opens the valve device when a pressure detected by the pressuresensing device reaches an upper limit value.

Another specific configuration of the cleaning chemical liquiddistribution unit is as follows. The header is located along thedirection in which the membrane units are arranged and along thehorizontal direction. The header is in communication with the cleaningchemical liquid supply source at the inlet thereof at one end thereof inthe pipe axis direction and with a return pipe at the outlet thereof atthe other end thereof in the pipe axis direction. The return pipe is incommunication with the inlet of the header or the cleaning chemicalliquid supply source.

In this configuration, during cleaning with the chemical liquid, asufficient amount of cleaning chemical liquid is allowed to flow to theheader. The cleaning chemical liquid flowing through the header flowsthrough the header and then the branch pipes to the collecting headersof the respective membrane units. Part of the cleaning chemical liquidreturns to the inlet of the header or the cleaning chemical liquidsupply source through the return pipe.

Since the cleaning chemical liquid circulates through the header and thereturn pipe, the velocity head is inhibited from being nonuniformlydistributed. This prevents a possible nonuniform branch pipe inletsupply pressure caused by the nonuniform distribution of the velocityhead. Furthermore, the “pushing force”, the first force, actssubstantially uniformly over the entire length of the header touniformize the branch pipe inlet supply pressure among all the branchpipes. The cleaning chemical liquid can thus be uniformly distributed toand contacted with the plurality of membrane units through the headerand the branch pipes.

In particular, when the branch pipes are connected to the header in thehorizontal direction, the gravity does not act as the force pushing thecleaning chemical liquid toward the inlet of each branch pipe. Instead,only the pressure head (the supply pressure of the pump or the like)acts as the “pushing force”, the first force. The cleaning chemicalliquid can thus be uniformly distributed to and contacted with theplurality of membrane units through the header and the branch pipes.

The header is preferably located at the liquid level position or abovethe liquid level position in the reaction vessel in the horizontaldirection. However, the header may be located below the liquid level inthe reaction vessel. Furthermore, each branch pipe is preferablyconnected to the header in the horizontal direction. However, the branchpipe may be connected on a slant to the header.

When the header is positioned above the liquid level in the reactionvessel, the cleaning chemical liquid flows through the header in anon-full condition in which a gas phase is present in the upper area ofthe header. The cleaning chemical liquid thus flows through each branchpipe to the collecting header of the corresponding membrane unit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of a large scale membrane separating installationaccording to an embodiment of the present invention;

FIG. 2 is a side view of the large scale membrane separatinginstallation according to the embodiment;

FIG. 3 is a sectional view taken along line a-a in FIG. 1 illustratingdifferences in effects between the embodiment of the present inventionand conventional embodiments;

FIGS. 4( a), (b) and (c) are graphs showing differences in effects amongthe configurations shown in FIG. 3;

FIG. 5 is an enlarged side view showing an essential part of the largescale membrane separating installation according to the embodiment ofthe present invention;

FIG. 6 is an enlarged front view showing an essential part of the largescale membrane separating installation according to the embodiment;

FIGS. 7( a) and (b) are enlarged side views showing an essential part ofa large scale membrane separating installation according to anotherembodiment of the present invention, wherein FIG. 7( a) shows that aheader is located above a liquid level and FIG. 7( b) shows that theheader is located at the liquid level;

FIG. 8 is a schematic diagram showing a large scale membrane separatinginstallation according to another embodiment of the present inventionwherein a header is located below the liquid level inside a vessel;

FIG. 9 is a schematic diagram showing a large scale membrane separatinginstallation according to another embodiment of the present inventionwherein a header is located below the liquid level outside the vessel;

FIG. 10 is a front view of a large scale membrane separatinginstallation according to another embodiment of the present invention;

FIG. 11 is a plan view of a large scale membrane separating installationaccording to another embodiment of the present invention;

FIG. 12 is an enlarged side view showing an essential part of the largescale membrane installation device according to the embodiment shown inFIG. 11;

FIG. 13 is a diagram showing the configuration of a pressure controldevice according to another embodiment of the present invention;

FIG. 14 is a front view of a large scale membrane separatinginstallation according to another embodiment of the present invention;

FIG. 15 is a side view of the large scale membrane separatinginstallation according to the embodiment shown in FIG. 14;

FIG. 16 is a front view of an essential part of a large scale membraneseparating installation according to another embodiment of the presentinvention;

FIG. 17 is a diagram showing effects of the large scale membraneseparating installation according to the present invention;

FIGS. 18( a), (b) and (c) are diagrams illustrating differences ineffects between the embodiments of the present invention andconventional embodiments; and

FIG. 19 is a front view showing a conventional large scale membraneseparating installation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings. The basic configuration of large scale membrane separatinginstallations according to the present embodiments is similar to thatdescribed with reference to FIGS. 1, 2, and 16. Thus, theabove-described reference numerals are also used in the descriptionbelow, and the description of the basic configuration is omitted.

In FIGS. 5 and 6, a plurality of membrane units 1 is arranged in areaction vessel. A header 2 is located along a direction in which themembrane units 1 are arranged and along a horizontal direction. Aplurality of branch pipes 3 branching from the header 2 are arrangedparallel to one another. Each of the branch pipes 3 is connected to acollecting header (not shown) of a corresponding one of the membraneunits 1.

The header 2 forms a channel with a diameter (large diameter) requiredto ensure a smooth flow. Each of the branch pipes 3 forms a channel witha diameter smaller than that of the header 2. Each of the collectingheaders is provided for a casing of a corresponding one of the membraneunits 1.

The header 2 is located in the horizontal direction at a predeterminedheight h1 (for example, 1 m) from a liquid level in the reaction vessel.Each branch pipe 3 is inclined with a falling gradient from the header 2toward the corresponding collecting header at a predetermined angle X(0<X<90°).

In this configuration, during cleaning with a chemical liquid, asubmerged pump 12 and a transfer pump 16 are driven to allow apredetermined amount of cleaning chemical liquid to flow through theheader 2. At this time, since the header 2 is positioned above theliquid level in the reaction vessel, the cleaning chemical liquid flowsthrough the header in a non-full condition, that is, with a gas phasepresent in an upper area of the header 2. The cleaning chemical liquidthen flows uniformly through the branch pipes 3 to the collectingheaders of the respective membrane units.

At this time, to allow the cleaning chemical liquid to flow uniformlyinto the branch pipes, it is essential to uniformize a branch pipe inletsupply pressure in the header 2. Thus, in the present embodiment, withthe “gravity” and “velocity head” taken into account, the branch pipes 3are inclined at 45° to the header 2 to allow a force resulting from the“gravity” and “velocity head” to contribute to uniformizing the branchpipe inlet supply pressure.

That is, the rate of gravity in the total force of the gravity and a“pushing force”, a first force, to which the gravity is added, isadjusted by the inclination of the branch pipes, to inhibit the flow ofan excessive amount of cleaning chemical liquid into the branch pipeslocated closer to the start point (inlet) of the header 2, while thecleaning chemical liquid is swept away toward the terminal of the header2 by a “sweeping force”, a second force. Thus, the branch pipe inletsupply pressure is uniformized such that the cleaning chemical liquidflows uniformly into all the branch pipes 3.

The predetermined inclination of each branch pipe 3 required touniformize the branch pipe inlet supply pressure is determinedmathematically or on the basis of the rule of thumb taking into accountthe amount of cleaning chemical liquid supplied and flowing into theheader 2, the supply pressure, the flow velocity of the cleaningchemical liquid flowing through the header 2, the ratio of the headerdiameter to the branch pipe diameter, the pipeline resistance, thegravity, and the like.

That is, the branch pipe inlet supply pressure can be uniformized bysetting the predetermined inclination of each branch pipe 3 on the basisof the above-described elements. The predetermined inclination of thebranch pipe 3 can be set on the basis of conditions such as the numberof the membrane units 1, the number of membrane cartridges in each ofthe membrane units 1, and the diameter of the piping.

The branch pipe inlet supply pressure in the header 2 can also beuniformed by a configuration shown in FIGS. 7( a) and 7(b). In FIGS. 7(a) and 7(b), the header 2 is located along the direction in which themembrane units 1 are arranged and along the horizontal direction. Theplurality of branch pipes 3 are arranged parallel to one another. Theheader 2 is positioned at or above the liquid level in the reactionvessel. Each of the branch pipes 3 is connected to the top of the header2. The branch pipe 3 has an overflow portion 21 at a predeterminedheight position h3 above the header 2.

The overflow portion 21 of the branch pipe 3 is located at thepredetermined height position h3. The predetermined height position h3is set on the basis of the pressure head generated in the cleaningchemical liquid in the header by the supply pressure of the cleaningchemical liquid (the supply pressure of a pump). This results in thepresence of a gas phase in an upper area of the overflow portion 21 whenthe cleaning chemical liquid flows through the overflow portion 21.

Furthermore, the distance from the liquid level to the header 2 is atmost 0.2 m, preventing a siphon from being established in the branchpipe 3. The siphon may vary the flow rate among the branch pipes 3.

Furthermore, the overflow portion 21 may have an air open portion 21 aincluding an on-off valve 21 b. The air open portion 21 a and the on-offvalve 21 b are not necessarily required for the overflow portion 21.

In this configuration, the cleaning chemical liquid supplied to theheader 2 during cleaning with the chemical liquid flows so as to fillthe header 2. The cleaning chemical liquid flowing through each branchpipe 3 overflows through the overflow portion 21 and flows to thecollecting header of the corresponding membrane unit 1. The gas phase ispresent in the upper area of the overflow portion 21 in the branch pipe3.

At this time, the interior of the header 2 is full of the cleaningchemical liquid to increase the effective channel cross section (thesubstantial channel cross section over which the cleaning chemicalliquid flows) of the header 2 compared to the effective channel crosssection observed in the non-full condition. This inhibits a possiblenonuniform branch pipe inlet supply pressure resulting from the velocityhead.

Since the branch pipe 3 has the overflow portion 21 at the position ofthe predetermined height h3 from the header 2, the gravity does not actas the “pushing force” pushing the cleaning chemical liquid toward aninlet of the branch pipe. Instead, only the pressure head (in this case,the supply pressure of the pump) acts as the “pushing force” pushing thecleaning chemical liquid toward the branch pipe inlet to lift thecleaning chemical liquid to the overflow portion 21 against the gravity.The cleaning chemical liquid then overflows through the overflow portion21.

Consequently, the branch pipe 3 forms the overflow portion 21 at thepredetermined height position h3 above the header 2. The cleaningchemical liquid can thus be uniformly distributed to and contacted withthe plurality of the membrane units 1 through the header 2 and thebranch pipes 3.

Furthermore, when the overflow portion 21 has the air open portion 21 aand the on-off valve 21 b, the on-off valve 21 b is open during cleaningwith the chemical liquid. In this condition, the pressure head in thecleaning chemical liquid supplied to the header 2 during cleaning withthe chemical liquid lifts the cleaning chemical liquid to the overflowportion 21 against the gravity. The cleaning chemical liquid thusoverflows through the overflow portion 21 of each branch pipe 3 with thegas phase present in the upper area of the overflow portion 21. Thecleaning chemical liquid then flows to the collecting header of thecorresponding membrane unit 1.

At this time, the overflow portion 21 is open to the air at the air openportion 21 a, preventing the gas phase in the overflow portion 21 fromcausing air lock. The cleaning chemical liquid flows naturally downwardthrough each branch pipe 3 under the atmospheric pressure to thecollecting header of the corresponding membrane unit 1. The cleaningchemical liquid can thus be uniformly distributed to and contacted withthe plurality of membrane units 1 through the header 2 and the branchpipes 3.

The branch pipe inlet supply pressure in the header 2 can also beuniformized by a configuration shown in FIGS. 8 and 9. In FIGS. 8 and 9,the header 2 is located along the direction in which the membrane units1 are arranged and along the horizontal direction. The plurality ofbranch pipes 3 are arranged parallel to one another. The header 2 ispositioned below the liquid level in the reaction vessel at apredetermined distance h4 therefrom. The header 2 may be located belowthe liquid level in the reaction vessel as shown in FIG. 8 or locatedoutside the reaction vessel as shown in FIG. 9. In this case, each ofthe branch pipes 3 may be connected at any angle to the header 2.

In this configuration, the cleaning chemical liquid supplied to theheader 2 during cleaning with the chemical liquid flows to the branchpipes 3 so as to fill the header 2. The cleaning chemical liquid thenflows through the branch pipes 3 to the collecting headers of therespective membrane units.

At this time, since the header 2 is positioned below the liquid level inthe reaction vessel, the interior of the header 2 is full of thecleaning chemical liquid. Consequently, the effective channel crosssection (the substantial channel cross section over which the cleaningchemical liquid flows) of the header 2 increases compared to theeffective channel cross section observed in the non-full condition. Thisinhibits a possible nonuniform branch pipe inlet supply pressureresulting from the velocity head.

Furthermore, the water head corresponding to the distance h4 from theliquid level to the header 2 acts as a back pressure. Thus, the gravitydoes not act as the “pushing force” pushing the cleaning chemical liquidtoward the branch pipe inlet. Instead, only the pressure head (in thiscase, the supply pressure of the pump) acts as the “pushing force”, thefirst force. The cleaning chemical liquid can thus be uniformlydistributed to and contacted with the plurality of membrane unitsthrough the header 2 and the branch pipes 3.

In this case, the channel cross section A of the header 2, the channelcross section B of each branch pipe 3, and the number of the branchpipes 3 desirably satisfy the relationship A>B×N×0.2.

The branch pipe inlet supply pressure in the header 2 can also beuniformized by a configuration shown in FIG. 10. In FIG. 10, the header2 is located along the direction in which the membrane units 1 arearranged and along the horizontal direction. The header 2 is positionedat or above the liquid level. The header 2 is divided into a pluralityof short pipes 2 a each corresponding to a predetermined number ofmembrane units 1. Each of the short pipes 2 a is in communication with acleaning liquid supply pipeline 11 at a central part thereof in a pipeaxis direction. The branch pipes 3 can be connected to the header 2either in the horizontal direction or in a vertical direction or in aninclining direction.

In this configuration, the cleaning chemical liquid supplied to theheader 2 during cleaning with the chemical liquid flows through thebranch pipes 3 to the collecting headers of the respective membraneunits. At this time, since the header 2 is divided into the short pipes2 a and each header 2 is in communication with the cleaning liquidsupply pipeline 11 at the central part thereof in the pipe axisdirection, the liquid level and velocity head in the short pipes 2 a ofeach header 2 are inhibited from being nonuniformly distributed. Thecleaning chemical liquid can thus be uniformly distributed to andcontacted with the plurality of membrane units 1 through the header 2and the branch pipes 3.

The branch pipe inlet supply pressure in the header 2 can also beuniformized by a configuration shown in FIGS. 11 and 12. In FIGS. 11 and12, the plurality of membrane units 1, located in the reaction vessel,are circularly arranged. The header 2 is located at an equal distance inthe vertical direction from the membrane units 1, that is, at a centralposition of the membrane units 1. The header 2 is located at or abovethe liquid level. The branch pipes 3 are arranged so as to branchradially from the header 2 and connected to the collecting headers ofthe respective membrane units 1.

In this configuration, the cleaning chemical liquid supplied to theheader 2 during cleaning with the chemical liquid flows through thebranch pipes 3 to the collecting headers of the respective membraneunits 1. At this time, since the cleaning chemical liquid flows into thebranch pipes 3 branching radially from the header 2, the possiblenon-uniform distribution of the velocity head in the header 2 isavoided. The pressure in the header 2 acts uniformly on the branch pipes3, so that the cleaning chemical liquid can be uniformly distributed toand made in contact with the plurality of membrane units 1 through theheader 2 and the branch pipes 3.

Furthermore, as shown in FIG. 13, a pressure control device 41maintaining the pressure at less than or equal to a set value may beprovided at the terminal of the header 2 in order to prevent anexcessive pressure from acting on the membrane units 1 through theheader 2 and the branch pipes 3. In this description, the header 2 ispositioned above the liquid level. However, the header 2 may be locatedbelow the liquid level.

The pressure control device 41 is made up of a pressure sensing device42 that measures the pressure at the terminal the header 2, a valvedevice 43 installed at a position of a predetermined height h5 (forexample, 2 m) from the liquid level in communication with the header 2,a control device 44 that opens the valve device 43 when the pressuredetected by the pressure sensing device 42 reaches an upper limit value,and a valve device 45 provided in the cleaning liquid supply pipeline11. A second selector valve 10 may be configured so as to vary the flowrate instead of providing the valve device 45.

In this configuration, the design of the header 2 and branch pipes 3 andthe total inflow amount of cleaning chemical liquid are adjusted suchthat for example, the pressure in each branch pipe 3 is set to about 0to 20 kPa. If the inflow of the liquid undergoes a high resistance owingto unexpected dirt or the like in the membrane unit, injection at astandard flow rate may result in an excessive pushing force pushing thecleaning chemical liquid toward the inlet of the branch pipe. Then, whenthe detected pressure measured by the pressure sensing device 42 of thepressure control device 41 reaches the upper limit value (which is setat, for example, 20 kPa), the control device 44 controllably opens andcloses the valve devices 43 and 45 to release the excessive pressure.This prevents the excessive pressure from acting on the correspondingmembrane unit 1, to control the flow rate.

The branch pipe inlet supply pressure in the header 2 can also beuniformized by a configuration shown in FIGS. 14 and 15.

The header 2 is located along the direction in which the membrane units1 are arranged. The branch pipes 3 branching from the header 2 areconnected to the collecting headers of the respective membrane unit 1(not shown). The header 2 is located along the direction in which themembrane units 1 are arranged and along the horizontal direction. Theheader 2 is positioned at the liquid level in the reaction vessel or ata predetermined height from the liquid level. The branch pipes 3 areconnected to the header 2 in the horizontal direction.

The header 2 is in communication with the cleaning liquid supplypipeline 11 at the start point (inlet) thereof in the flow direction. Afirst end of a return pipe 51 is connected to the terminal (outlet) ofthe header 2 in the flow direction. A second end of the return pipe 51is in communication with the inlet of the header 2. An on-off valve 52is provided in the return pipe 51. The second end of the return pipe 51can be connected to the cleaning liquid supply pipeline 11. Furthermore,as shown in FIG. 16, the first end of the return pipe 51 may beconnected to the branch pipe 3 positioned at the terminal of the header2.

In this configuration, during a filtration operation, a valve 4 and afirst selector valve 8 are open. The on-off valve 52 and the secondselector valve 10 are closed. In this condition, a treated water pump 9is driven to apply a suction pressure to the membrane cartridges in eachof the membrane units 1. The suction pressure is used as a drivingpressure to perform a filtration operation using the membranecartridges. A filtrate having permeated a filtration membrane flowsthrough a tube (not shown) into the collecting header (not shown). Thefiltrate further flows through the branch pipe 3 to the header 2 andthen through a filtrate recovery pipeline 6 into the treated water tank7.

During a backwashing operation, the valve 4, the second selector valve10 and the on-off valve 52 are open, and the first selector valve 8 isclosed. In this condition, the submerged pump 12 is driven to supplytreated water in the treated water tank 7. The transfer pump 16 isdriven to supply the cleaning chemical liquid.

At this time, since the header 2 is positioned above the liquid level inthe reaction vessel, the cleaning chemical liquid flows through theheader in the non-full condition, that is, with the gas phase present inthe upper area of the header 2. The cleaning chemical liquid flowsthrough the header 2 and the branch pipes 3 to the collecting headers ofthe respective membrane units. Part of the cleaning chemical liquidreturns to the inlet of the header 2 or the cleaning liquid supplypipeline 11 through the return pipe 51. The cleaning chemical liquidthus circulates through the header 2 and the return pipe 51.

This inhibits the velocity head from being nonuniformly distributed,preventing a possible nonuniform branch pipe inlet supply pressureresulting from the nonuniform distribution of the velocity head. The“pushing force”, the first force, acts substantially uniformly over theentire length of the header 2 to uniformize the branch pipe inlet supplypressure among all the branch pipes 3. The cleaning chemical liquid canthus be uniformly distributed to and contacted with the plurality ofmembrane units 1 through the header 2 and the branch pipes 3.

Furthermore, since the branch pipes 3 are connected to the header 2 inthe horizontal direction, the gravity does not act as the “pushingforce” pushing the cleaning chemical liquid toward the branch pipeinlet. Instead, only the pressure head (in this case, the supplypressure of the pump) acts as the “pushing force”, the first force. Thecleaning chemical liquid can thus be uniformly distributed to andcontacted with the plurality of membrane units 1 through the header 2and the branch pipes 3.

Embodiments

Description will be given below of embodiments based on calculationsaccording to the present invention.

1. The cleaning chemical liquid flows through the header 2 in thenon-full condition (see FIG. 17).

1-1. The branch pipe 3 is located in the vertical direction as shown inFIG. 18( a).

When Q=the flow rate in each branch pipe, a=the channel cross section ofthe branch pipe, v=the flow velocity in the branch pipe, V=the flowvelocity in the header, A=the channel cross section actually occupied bythe cleaning chemical liquid in the header, h=the water level in theheader, and g=9.8 m/s², and it is assumed that the installation has asingle branch pipe, then Q=v×a=sqrt(2gh)×a=VA.

Thus, determining the channel cross section a of the branch pipe and theflow rate Q in the branch pipe allows the water level h in the header 2to be determined according to the formula:

h=Q×Q/(a×a×2g).

Here, when the branch pipe diameter is assumed to be 50 mm and the flowrate in the branch pipe is assumed to be 60 litters/min., a=0.002 m²,Q=0.001 m³/sec, and v=0.5. The water depth in the header 2 ish=0.001×0.001/(0.002×0.002×2×9.8)=0.0128 m=12.8 mm.

a) The header 2 has a diameter of 150 mm.

In this case, A=0.00073 m² and V=Q/A=1.37 m/s.

Thus, to allow the cleaning chemical liquid to flow uniformly throughthe 10 branch pipes 3, it is necessary to set the inflow velocity to10V=13.7 m/s.

In a free fall from a height of 1 m, the cleaning chemical liquid flowsin at a flow velocity of 4.4 m/s. Thus, for example, when the cleaningchemical liquid is allowed to flow through the header 2 of diameter 150mm at the same flow rate, A=10Q/4.4=0.01/4.4=0.00227.

In this case, h=0.027 and v=sqrt(2gh)=sqrt(2×9.8×0.027)=0.73.

The flow rate Q1 in the first branch pipe: Q1=0.73×0.002=0.00146. Thecleaning chemical liquid flows through the first branch pipe at a flowrate about 1.5 times as large as a design value.

The flow rate Q2 in the second branch pipe:

Q2 is equivalent to Q1 on the basis of A=0.009854/4.4=0.00224. As aresult, the flow rates in the first two branch pipes amount to nearly30% of the total value. Thus, the cleaning chemical liquid does not flowto the last branch pipe.

1-2. The branch pipe 3 is obliquely located at an angle of 45° to theheader as shown in FIG. 18( b).

As in the above-described case, Q=v×a=sqrt(2gh)×a=VA and h=Q×Q/(a×a×2g).

Here, when the branch diameter is assumed to be 50 mm and the flow rateis assumed to be 60 litters/min., a=0.002 m², Q=0.001 m³/sec, and thewater depth h in the header 2=0.001×0.001/(0.002×0.002×2×9.8)=0.0128m=12.8 mm.

a) The header 2 has a diameter of 150 mm.

The center of the branch pipe is 0.022 m away from the bottom surfacethereof.

The total water depth is 0.0128+0.022=0.0348.

In this case, A=0.00298 m² and V=Q/A=0.336 m/s.

This is the velocity at which the cleaning chemical liquid flows in afree fall from a height of about 60 cm.

Therefore, when the header 2 has a diameter of 150 mm, injection from aheight of about 60 cm allows the cleaning chemical liquid to beuniformly fed into each branch pipe.

1-3. The branch pipe 3 is located in the horizontal direction as shownin FIG. 18( c).

As in the above-described case, Q=v×a=sqrt(2gh)×a=VA and h=Q×Q/(a×a×2g).

Here, when the branch diameter is assumed to be 50 mm and the flow rateis assumed to be 60 litters/min., a=0.002 m², Q=0.001 m³/sec, and thewater depth h in the header 2=0.001×0.001/(0.002×0.002×2×9.8)=0.0128m=12.8 mm.

a) The header 2 has a diameter of 150 mm.

The center of the branch pipe is 0.075 m away from the bottom surfacethereof.

The total water depth is 0.0878.

In this case, A=0.0101 m² and V=Q/A=0.10 m/s.

Thus, to allow the cleaning chemical liquid to flow uniformly throughthe 10 branch pipes 3, it is necessary to set the inflow velocity to10V=1.0 m/s (from a height of 0.05 m in a free fall).

In a free fall from a height of 1 m, the cleaning chemical liquid flowsin at a flow velocity of 4.4 m/s. Thus, for example, when the cleaningchemical liquid is allowed to flow through piping of diameter 100 mm atthe same flow rate, A=10Q/4.4=0.01/4.4=0.00227.

In this case, the total water depth is 0.0325<0.075. Thus, the waterlevel is lower than the center (0.075) of the branch pipe, preventingthe cleaning chemical liquid from flowing out of the first branch pipe.

As a result, the water level increases sequentially from the terminal ofthe header, increasing the flow rate in the corresponding branch pipesabove the design value. Furthermore, to bring the whole header into thefull condition, it is necessary that v=sqrt(2gh)=1 m andQ=10a×sqrt(2gh)=10×0.002×=0.2 m³/sec=120 ml/min. That is, the flow rateneeds to be doubled, preventing the header from becoming full.Therefore, the water level increases sequentially from the terminal ofthe header to increase the flow rate in the corresponding branch pipes,while reducing the flow rate in several branch pipes located closer tothe start point.

1. A large scale membrane separating installation comprising a plurality of membrane units arranged in a reaction vessel and a plurality of branch pipes branching from a header and connected to collecting headers of the respective membrane units so that a cleaning chemical liquid flows through the header and the branch pipes to the respective membrane units, wherein a cleaning chemical liquid distribution unit comprises the header and the branch pipes each having a smaller diameter than the header, and the cleaning chemical liquid distribution unit comprises a pipeline configuration uniformly distributing the cleaning chemical liquid to be supplied to the membrane units.
 2. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit adjusts a first force resulting mainly from a pressure head and a second force resulting mainly from a velocity head, and wherein the first force acts as a force that pushes the cleaning chemical liquid in the header toward an inlet of each of the branch pipes, and the second force acts as a force that sweeps away the cleaning chemical liquid in the header in a pipe axis direction.
 3. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, mounting axes of the branch pipes are arranged parallel to one another, the header is positioned above a liquid level in the reaction vessel, and each of the branch pipes is inclined with a falling gradient from the header toward the collecting header at a predetermined angle.
 4. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, mounting axes of the branch pipes are arranged parallel to one another, the header is positioned above a liquid level in the reaction vessel, and each of the branch pipes has an overflow portion at a predetermined height position above the header.
 5. The large scale membrane separating installation according to claim 4, wherein an air open portion is provided in the overflow portion of each branch pipe.
 6. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, mounting axes of the branch pipes are arranged parallel to one another, the header is positioned below a liquid level in the reaction vessel, and each of the branch pipes is connected to the header below the liquid level.
 7. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, mounting axes of the branch pipes are arranged parallel to one another, the header is positioned at or above a liquid level, and the header is divided into a plurality of short pipelines each of which is located for every predetermined number of membrane units and each of which is in communication with a cleaning chemical liquid supply source at a central part thereof in a pipe axis direction.
 8. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located at an equal distance in a vertical direction from the plurality of membrane units circularly arranged in the reaction vessel, the branch pipes branching from the header are radially arranged, and the header is positioned at or above a liquid level.
 9. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, mounting axes of the branch pipes are arranged parallel to one another, the header is in communication with a cleaning chemical liquid supply source at an inlet of the header at one end thereof in a pipe axis direction, and the header has a pressure control device provided at the other end thereof in the pipe axis direction to maintain a pressure at less than or equal to a set value.
 10. The large scale membrane separating installation according to claim 9, wherein the pressure control device comprises a pressure sensing device, a valve device that is in communication with the header, and a control device that opens the valve device when a pressure detected by the pressure sensing device reaches an upper limit value.
 11. The large scale membrane separating installation according to claim 1, wherein the cleaning chemical liquid distribution unit is configured such that the header is located along a direction in which the membrane units are arranged and along a horizontal direction, the header is in communication with a cleaning chemical liquid supply source at an inlet thereof at one end thereof in a pipe axis direction and with a return pipe at an outlet thereof at the other end thereof in the pipe axis direction, and the return pipe is in communication with the inlet of the header or the cleaning chemical liquid supply source. 